The Neurobiology of Thirst and Sodium Appetite

Abstract

Water and salt are essential to life on earth. Multicellular terrestrial organisms must obtain water and salt, circulate them to the cells, and minimize loss and excess. In mammals, the nervous system controls the responses to these problems. It responds to internal and external sensory signals, particularly when a deficit exists in water, sodium, or both. The absolute values for water and electrolytes are less important than the relative ratios. These ratios are maintained within strict limits by physiological and endocrinological responses. Nevertheless, some loss is inevitable. For most mammals, behavioral responses are the only means of redressing these losses. The animals must seek out and consume water, sodium, or both. The behavioral responses are not necessarily fixed but can vary with experience and circumstance. For this reason, we invoke psychological constructs, motivations – thirst for water, sodium appetite for salt deficit – to account for the directed attention and energy expenditure characterizing the often learned, complex behaviors that lead the animal to water or salt. Besides the brain, the major organs of hydromineral balance are the kidneys and the cardiovascular system. The most important hormones involved are angiotensin and aldosterone. These internal systems and behavior all depend on the hypothalamic-preoptic continuum to coordinate their activity. To some degree, the physiological and endocrinological responses operate independently, but the preoptic and hypothalamic areas are required to perfect their effects. Behavior also needs these brain areas but it requires the rest of the brain to seek out water and salt in the environment. The clinical importance of hydromineral balance cannot be overstated. Humans have only a limited ability to store water. Thus, acute dehydration quickly can become fatal. Regardless of the cause, dehydration includes some sodium loss. Replacing too much water can disrupt the hydromineral balance beyond the ability of the physiological and endocrinological systems to compensate. This also can rapidly become fatal. In many patients, the physician must substitute their own behavior to ensure that these vital substances are replenished correctly and in balance.